Digital flight data recording systems (DFDRS) provide nonvolatile recording of sensed flight data signal information for post flight analysis of aircraft performance. This occurs through reconstruction of the recorded real time flight information. The information is first compressed by data acquisition circuitry (DAU) and the most significant sensed data values are formatted into message frames and transmitted from the DAU to a nonvolatile recorder memory.
The data is transmitted between the DAU and recorder memory through a serial digital signal communication link defined by the Electronics Industry of America (EIA) full duplex RS422 specification, which requires a pair of two wire transmission lines for simultaneously transmitting data in each direction. In the DFDRS, however, data transmission only occurs in one direction at a time, i.e. half duplex. The RS422 specification requires the data to be encoded in a differential format in which the voltage of one line with respect to the other is plus or minus five volts, all of which is known to those skilled in the art.
The fidelity of the recorded information is critical for resolute waveform reconstruction. Since the communications interface between the DAU and recoder is necessary to the storing of the information, the reliability of the communications circuitry is critcal to the overall reliability of the recording system.